1. Field of the Invention
This invention relates to a thermal head for thermally making a stencil for use in a stencil printer, a method of manufacturing such a thermal head and a thermal stencil making apparatus using such a thermal head.
2. Description of the Related Art
There has been known a stencil making apparatus having a stencil making section such as shown in FIG. 5. The stencil making section comprises a platen roller 3 having a metal support shaft 3a which is supported for rotation on a side frame (not shown) at its opposite ends and a thermal head 2 which is pressed against the platen roller 3 and is moved away from the platen roller 3 by a head pressing mechanism (not shown).
The thermal head 2 comprises a heat radiating plate 21, a ceramic substrate 22 fixed to the heat radiating plate 21, and a glaze layer 23 which is fixed to the surface of the ceramic substrate 22 and functions as a heat accumulating layer. An array of resistance heater elements 24 is formed on the surface of the glaze layer 23. The heater elements 24 are connected to electrodes and a drive circuit (which are not shown) and are selectively energized to thermally perforate a stencil material 4.
When making a stencil by imagewise perforating a stencil material 4, the stencil material 4 is fed between the thermal head 2 and the platen roller 3, and then the thermal head 2 is pressed against the platen roller 3 with the stencil material 4 intervening therebetween. With the thermal head 2 thus kept in a close contact with the stencil material 4, the resistance heater elements 24 are selectively energized to thermally perforate the stencil material 4. Thereafter, the platen roller 3 is rotated to bring the thermal head 2 in contact with another part of the stencil material 4 and the resistance elements 24 are selectively energized again to thermally perforate the stencil material 4. By repeating these steps, a stencil master is made.
There has been a problem that, since the platen roller 3 is supported only at opposite ends of the support shaft 3a, the platen roller 3 is deflected at the middle thereof as shown in FIG. 5 in an exaggerated scale, whereas the thermal head 2 is normally formed of highly rigid materials and is hardly deflected. The thermal head 2 cannot be pressed against the platen roller 3 under a sufficient pressure near the middle of the platen roller 3.
FIG. 6 shows the measured value of the pressure acting between the thermal head 2 and the platen roller 3 per unit area when the thermal head 2 is pushed toward the platen roller 3 under a predetermined force by the head pressing mechanism. As can be seen from FIG. 6, the pressure acting between the thermal head 2 and the platen roller 3 is low near the middle of the platen roller 3 as compared with near the ends of the same, which results in a higher probability of generating defective perforations near the middle of the stencil.
When the pressure under which the thermal head 2 is pressed against the platen roller 3 is reduced in order to suppress deflection of the platen roller 3, the probability of generating defective perforations is increased over the entire area of the stencil, which can result in deterioration in printing density.
Recently, there is a tendency to make larger the stencil, and, as the size of the stencil increases, the platen roller 3 must be larger in length, which results in an increased probability of generating defective perforations near the middle of the stencil.
There has been proposed a thermal stencil making apparatus in which a thermal head convex near the middle is used in order to suppress reduction in pressure between the platen roller 3 and the thermal head 2 due to deflection of the platen roller 3.
Conventionally, since such a convex thermal head has been formed, for instance, by pressing a convex heat radiating plate and fixing a ceramic substrate provided with resistance heater elements to the convex heat radiating plate, the degree of convexity of the thermal head obtained is governed by the state in which the ceramic substrate is fixed to the heat radiating plate, which makes it very difficult to obtain a desired degree of convexity of the thermal head.
Further, there has been known a convex thermal head which is formed by fixing a ceramic substrate to a flat heat radiating plate and then applying a pressure to the assembly of the heat radiating plate and the substrate to deform the assembly into a convex. However this method is disadvantageous in that it is necessary to control the pressure to be applied to the assembly according to the state in which the ceramic substrate is fixed to the heat radiating plate and accordingly it is very difficult to control the pressure to obtain a desired degree of convexity of the thermal head.
Further, intention to quickly deform the assembly of the heat radiating plate and the substrate into a convex is apt to result in breakage of the ceramic substrate and/or the glaze layer on the substrate. When the assembly is to be deformed by application of a pressure for a long time, though fear of breakage of the ceramic substrate and/or the glaze layer on the substrate is suppressed, productivity of the thermal head lowers and accordingly the manufacturing cost of the thermal head increases.